A conventional control system of a device for controlling a wheel loader mainly for loading work is illustrated in FIG. 11.
With reference to FIG. 11, the output of an engine E is transmitted to a torque converter TC and a gear G and the output transmitted to the gear G drives hydraulic pumps P.sub.1 and P.sub.2 of a fixed capacity.
A bucket operation pilot valve AL is operated to actuate a bucket main operation valve AV to thereby turn a bucket A by way of a bucket cylinder AC, so that the bucket A tilts rearward or dumps forward.
A boom operation pilot valve BL is operated to actuate a boom main operation valve BV to thereby turn a boom B by way of a boom cylinder BC, so that the boom B lifts upward or lowers downward.
Designated at PP is a pilot pump.
The performance of the hydraulic pumps P1 and P2 of a fixed capacity in the conventional control system of a working machine is illustrated in FIG. 12(a).
In the same figure, P.sub.1, P.sub.2, Q.sub.1 and Q.sub.2 represent respectively the oil pressures and the flow rates of the hydraulic pumps P1 and P2.
A rectangle surrounded by O-P.sub.2 -P.sub.2 Q.sub.2 point-Q.sub.2 is a region where the hydraulic pump P.sub.2 alone operates and a hatching portion surrounded by O-P.sub.2 -P.sub.2 Q.sub.2 point-P.sub.1 Q.sub.2 point-P.sub.1 Q.sub.1 point-Q.sub.1 is a region where both the hydraulic pumps P.sub.1 and P.sub.2 operate.
FIG. 12(b) is a graph showing a torque curve of the output of an engine wherein T.sub.EM is a torque curve at the full throttling time, T.sub.EM G is a torque curve when an electronic control type governor operates, T.sub.T is an absorption torque curve of a torque converter, N.sub.C1 is an engine speed at the point where the absorption torque curve T.sub.T intersects the torque curve T.sub.EM G, T.sub.PA is an average hydraulic pump torque when the oil pressure in a hydraulic circuit of a working machine is low, T.sub.A1 is a torque at the point where the absorption torque curve T.sub.T intersects the curve of the torque obtained by subtracting T.sub.PA from T.sub.EM, T.sub.B1 is a torque at the point where the absorption torque curve T.sub.T intersects the curve of the torque obtained by subtracting T.sub.PB from T.sub.EM, and N.sub.B1 is an engine speed at that time.
Referring back to FIG. 12(a), designated at P.sub.A is a pressure at the point where the average hydraulic pump torque curve T.sub.PA at a low oil pressure intersects the vertical line connecting the P.sub.1 Q.sub.1 point to Q.sub.1 and P.sub.B is an oil pressure at the point where the average oil pressure pump torque curve T.sub.PB at a high oil pressure intersects the vertical line connecting the P.sub.2 Q.sub.2 point to Q.sub.2.
As is evident from FIGS. 12(a) and (b), according to the conventional control system of the working machine, the distribution of the output of the engine to the travel of the vehicle and the operation of the working machine are selected in two steps depending on the oil pressure, namely, a high pressure or a low pressure in the hydraulic circuit of the working machine.
That is, the hydraulic pump torque T.sub.PB, i.e. an oil pressure load T.sub.PB is reduced at the high pressure (e.g. at the excavating time) so that the torque curve T.sub.EM, i.e. the output T.sub.EM of the engine is much distributed to the travel torque T.sub.B1 and the hydraulic pump torque T.sub.PA, i.e. an oil pressure load T.sub.PA is increased at the low pressure (e.g. at the load lifting time) so that the output T.sub.EM of the engine is much distributed to the operation of the working machine, whereby the output of the engine can be effectively utilized.
In the conventional control system, since the engine torque T.sub.EM is fixed, the distribution of the torque at the travelling time and the working time is limited by the pump capacity (Q.sub.1 or Q.sub.1 +Q.sub.2) so that the output of the engine for the travel is difficult to be ideally set (actually an operator performs the throttle operation to thereby control the travel output).
There is such a serious problem in the conventional control system for selecting the pump capacity in two steps by the oil pressure alone that the variation of the output of the working machine, particularly the variation of the speed of the working machine occurs in the middle of working so that an unskilled operator can hardly operate the working machine.
There is another problem that the engine speed hardly increases when it is changed from a low speed to a speed burst and a heavy load working is required adding to that.
It is an object of the present invention to solve the problems set forth above.